Answers

When you use the colors in this manner they become declined as adjectives. The adjective in german when used this way declines into the same gender and number as the noun they are describing.

In your examples Auto is neutral singular, the adjectives in neutral singular (nominative) end in -es,hence gelb-es Auto/ rot-es Auto. It would do the same if instead of saying a color you said simply that it was a big (groß) car: groß-es Auto.

Blumen are plural so the adjective ends in -en: orang-en Blumen /schön-en Blumen.

Lila however is an exception, this color doesn't decline at all and that's why it wasn't corrected.

If an adjective is not preceded by any of the words mentioned above, it takes "strong" endings. Adjectives preceded by indefinite pronouns or numerals take strong endings. EXCEPTION: when using keine and alle, the adjective following these indefinite expressions take weak endings.

There are three neat tables in the book for each type of ending. You might want to make a some tables yourself to refer to.Weak endings:Nominative Singular -eAccusative Masculine Singular -enAccusative Feminine or Neuter Singular -eDative (all) -enGenetive (all) -enAll plurals -en